Surely ground and negative on a negative earth 12V vehicle are the same thing?

I am not so sure that hooking up 12V to the wires of the Aux fan is such a good thing. My understanding is that the fan is a variable speed unit and that the speed of fan is defined by a signal on one of the wires... Without knowledge of the type of control signal I think you could risk cooking the speed controller by thumping 12V down the wrong wire!